In high frequency circuit elements such as microwave integrated circuits, low dielectric materials are frequently employed for insulation of circuit components. For example, a microwave resonator generally comprises, as shown in FIG. 2, a dielectric resonator 1, a ceramic substrate 3 and a supporting member 2 of a low dielectric material interposed between them. To make a complete assembly, the entire unit is encapsulated in a metal case 5. Electromagnetic energy is supplied to the resonator 1 by strip lines 4 deposited on the substrate and electromagnetically coupled to the resonator by electromagnetic field H.
In such microwave devices, the unloaded Q of the resonant system is decreased by conductor loss. Thus, the smaller the leakage of the electric field through the supporting member, the greater the unloaded Q value of its resonant system. For this reason, a material for the supporting member is required to have a lower dielectric constant and have a smaller dielectric loss factor (tan .delta.). Up to now, forsterite is most widely used as a material for supporting members since it has a low dielectric constant of 6.5. However, the Q value of the forsterite is about 3000 at 10 GHz, so that there is an increasing demand for development of dielectric materials with a higher Q value.
On the other hand, alumina ceramics which are mainly used as a material for high frequency circuit boards have a high flexural strength of about 3000 kg/cm.sup.2, but they have a relatively high dielectric constant of about 9.0 to 9.5. When strip lines with a high impedance are to be formed on the alumina substrate, it is required to make the width of the strip lines small (generally, less than 1 .mu.m). This causes broken strip lines, resulting in an increase in the number of inferior microwave devices.
The impedance of strip lines formed on the ceramic substrate with a given thickness is, very roughly speaking, in inverse proportion to the width of the strip lines and the dielectric constant of the substrate. Thus, the impedance of the strip lines can be increased by use of materials with a lower dielectric constant, instead of the reduction in width of the strip lines. However, the low dielectric materials of the prior art such as, for example, forsterite (.epsilon.: 6.5) and steatite (.epsilon.: 5.5-7.5) can lead to a decrease in reliability of the microwave devices since their flexural strengths are less than 1500 kg/cm.sup.2 and approximately less than one-half of that of alumina ceramics.